DESCRIPTION The long-term objective of the proposed research is to elucidate the mechanisms of protein-protein binding mediated by the leucine-rich repeat (LRR) structural motif of the yeast CCR4 protein. Tandem LRR arrays form a conserved structural motif which mediates reversible high affinity protein- protein interactions. The expanding superfamily of LRR-containing proteins consists of at least 80 proteins which participate in the regulation of cell signaling and other diverse cellular processes. A number of LRR-containing proteins have been implicated in pathogenesis and immunity. Therefore, study of the mechanisms of LRR function could prove useful in the development of treatments for a variety of human diseases. Yet, for the majority of these proteins, detailed sequence-structure data and molecular models of protein-protein binding are not available. In the proposed study, the yeast CCR4-NOT complex will be used as a model containing protein to its cognitive ligand(s). The specific aims of this complex will be used as a model containing protein to its cognitive ligand(s). The specific aims of this project include i) mutagenesis of interstitial residues of the LRR region of CCR4 to define residues critical for physical and functional binding to protein ligands within the CCR4-NOT complex, ii) the identification of cognitive protein ligands which bind directly to CCR-LRR, and iii) the development of identification of cognitive protein ligands which bind directly to CCR-LRR, and iii) the development of a genetic system to identify mutations that restore or abrogate LRR-ligand interactions.